Melatonin is a hormone produced by the pineal gland, which is located at the back of the third ventri-cle of the brain. The secretion of melatonin is greatest during darkness and decreases when light enters the eye and the retina signals the hypothalamus. A recent discovery is that the retina also produces melatonin, which seems to indicate that the eyes and pineal gland work with the biological clock of the hypothalamus. In other mammals, melatonin helps regulate seasonal reproductive cycles. For people, melatonin definitely stimulates the onset of sleep and increases its duration Other claims, such as that melatonin strengthens the immune system or prevents cellular damage and aging, are without evidence as yet.
There are other organs that produce hormones that have only one or a few target organs. For example, the stomach and duodenum produce hormones that regu-late aspects of digestion and appetite. Adipose tissue produces the appetite-suppressing hormone leptin. The thymus gland produces hormones necessary for the normal functioning of the immune system, and the kidneys produce a hormone that stimulates red blood cell production.